In flight, there are two types of potential collision. The aircraft may either strike the ground or another craft forming part of the air traffic. The aeronautical industry is focusing its efforts on means of reducing and ultimately eliminating all future accidents associated with such collisions.
Regarding collisions with the terrain, the technological progress made notably in the development of digitized terrain files, in positioning accuracy thanks to “GPS” (Global Positioning System) locating and in the increase in processing power has made it possible to develop novel concepts making it possible to safeguard against these risks of collision with the ground in controlled flight, better known by the acronym CFIT which stands for “Controlled Flight Into Terrain”.
These concepts are based essentially on an extrapolation of the current trajectory of the aircraft and on a terrain database corresponding to the relief being flown over making it possible to predict these risks. In general, there are two types of terrain that present a risk of collision with the aircraft. The first type represents the areas of terrain representing an imminent danger to the craft. These are called “TAA”, which stands for “Terrain Alert Area”. The second type represents the areas of terrain representing a more distant danger to the craft. These are called “THD”, which stands for “Terrain Hazard Display”. These concepts are embodied in the form of a new generation of equipment intended for transport or business airplanes called “TAWS” standing for “Terrain Awareness and Warning System” or “GCAS” standing for “Ground Collision Avoidance System”. The “GCAS” system essentially comprises a central electronic computer linked on the one hand to the network of probes and feelers of the aircraft and on the other hand to the different displays on the instrument panel, as well as to the audible alarms arranged in the cockpit, by means of a data transmission bus. The probes are essentially sensors making it possible to determine the position of the craft relative to the ground, its altitude and its speed.
The displays concerned are the head-up viewing device and the piloting and navigation head-down screens such as the “Navigation Display” and “Primary Flight Displays”, screens denoted “HUD”, “ND” and “PFD”.
In the same way, current civilian aircraft are provided with traffic alert and collision avoidance systems, better known by their acronym “T-CAS”, which stands for “Traffic Alert and Collision Avoidance System”. This system is intended to significantly reduce the number of collisions between aircraft. It is mandatory on all craft exceeding a certain weight or transporting a certain number of passengers.
The “T-CAS” systems comprise at least one transponder, means of processing received data and at least one viewing means. An aircraft transponder is an onboard system which transmits a message concerning the aircraft when it receives an interrogation message. The message generally comprises a code identifying the craft and more or less detailed information on the position of the craft. In airplanes provided with viewing screens, the “TCAS” function is incorporated in the screen dedicated to navigation which is also called “Navigation Display” or ND. Generally, the “TCAS” system distinguishes two regions: a first region within which another aircraft may represent a risk of collision. If an aircraft appears in this region and represents a potential collision hazard, a traffic notification is emitted by the device called “Traffic Advisory” or “TA”. The latter informs the pilot that another airplane is located nearby but does not suggest any avoidance maneuver. The second region is situated in the immediate environment of the aircraft. In this case, if an airplane appears in this region, the situation is aggravated and the collision seems imminent, an audio message and a visual alert are produced by the device called “Resolution Advisory” or “RA”, indicating the airplane concerned and signaling the action to be performed by the pilot, namely either to maintain the current trajectory, or to climb, or to descend or even to monitor the vertical speed. The system is designed so that the “TCAS” of the other airplane recommends another maneuver. Very often, the “TCAS” tells the first airplane to climb and the second airplane to descend, which considerably increases the separation between the two craft. When the alert is ended, the system announces the end of the conflict.
In current civilian airplanes, a “GCAS” or “TCAS” alert triggers the following items:                a voice maneuver instruction message which may be, for example, “PULL-UP” which means “CLIMB” in the case of a “GCAS” alarm or even “CLIMB-CLIMB” which means “CLIMB STEEPLY” in the case of a “TCAS” alarm;        a display of the instruction on the “PFD” screen, for example a “PULL-UP” message in the case of a “GCAS” alarm or one or two red areas to be avoided and a green area in which to place the needle corresponding to the vertical speed of the craft in case of a “TCAS” alarm;        a display of data of “situation awareness” type is superimposed on the situation permanently present on the screen of the “ND”.                    In case of a “GCAS” alarm, this is the terrain area “TAA” calculated by the “TAWS” which is superimposed on the “THD” area already present and possibly on a map background concerning the most recent airplanes. This so-called 2.5D map background represents the terrain being flown over, the relief being represented in the form of shading.            In case of a “TCAS” alarm, this is the display of the threatening airplane in amber or in red depending on whether the alert level is of the “TA” or “RA” type.                        
When the aircraft has a head-up viewing device, the latter conventionally displays information concerning piloting or navigation. In a head-up viewing device, such information is collimated to infinity and projected by superimposition on the external landscape. An example of this type of representation is given in FIG. 1. The pilot therefore simultaneously sees a projected symbol system and the “real world” situated outside the craft. In this figure and the subsequent figures, the external rectangle represents the limits of the display field of the viewing device. There is conventionally, on the right of FIG. 2, an altitude scale 1 in feet, on the left a speed scale 2 in knots, in the center, the horizon 3, the flight director 4 and at the bottom of FIG. 2, the heading indicator 5 with the indication of the path to be followed, better known by the acronym “HSI” which stands for “Horizontal Situation Indicator”. As can be seen, this figure includes a large number of symbols that vary constantly according to the position of the craft. In nominal mode, this symbol system is perfectly suited to piloting and navigation. However, if a risk of collision occurs, in case of a “TCAS” alert only, currently, when a “HUD” is present, a guidance instruction is simply presented in the head-up display to avoid overloading the symbol system.
The drawback of the existing system is that a pilot using the guidance symbol system in head-up mode to modify the trajectory of the airplane cannot at the same time monitor the trend of the situation with regard to the threat originating the “TCAS” or “GCAS” alarm.